In several applications of measurement technology, the signals to be measured are very weak in size. For example, biomagnetic signals are among signals of this type. Weak signals are very susceptible to electromagnetic interferences produced by the environment for the interference signals to be perceived in the environment can be up to million times bigger compared to the biomagnetic signal being measured. Therefore, the sensors or other measuring devices that measure weak signals must be shielded very well against external interferences, or the effect of the interferences must be minimised in the region to be measured in some other manner.
One way of shielding sensitive measuring devices against electromagnetic interferences is to place near the measuring sensors sources which produce an electric and magnetic field and compensate the interferences. This enables one to produce in the region of the measuring sensors a resultant field enabling one to perceive a weak useful signal to be measured as the interference field and the compensating field cancel each other as well as possible.
Another way of protecting oneself from interferences is to place the measuring devices within a magnetically shielded room. The shielded room is made of a material that effectively suppresses the magnetic field caused by an external interference. The shielded room can be fixedly built in a desired place, or can be a so-called light-weight shielded room, which is built by assembling a shielding structure from separate elements about the measuring devices. Besides the materials of the elements, the joints between the wall elements are of importance from the standpoint of the propagating of the magnetic field, so specific attention must be paid to the structure of the joints.
Publication WO 03059030 discloses a structure of a wall element designed for a magnetically shielded room. The wall element has a so-called sandwich structure having an electrically conducting metal plate in the middle. The metal can consist e.g. of aluminum. The middlemost plate has on both sides thereof plates that are made of a different material than the middlemost plate, but are similar to one another. The edging plates have a high magnetic permeability and are typically made of μ metal. Furthermore, the structure comprises another thinner aluminum plate so that the order of the plates as viewed from the cross section of the element is as follows: a thinner aluminum plate, a μ metal plate, a thicker aluminum plate, a μ metal plate. Placed between the thicker aluminum plate and the μ metal plate is a thin layer of dielectric material. The plates are tightly attached to one another without air gaps. This structure yields a good magnetic protection against interferences.
Further, publication WO 03059030 describes a joint structure between two wall elements of a shielded room. The wall elements are placed a little apart from each other, and two strips are placed in the junction one on both sides of the joint. One strip consists of a thin layer of resilient material, of a layer of ferromagnetic material (such as μ metal) and of an aluminum layer. The ferromagnetic layer joins the μ metal parts of the elements to form a magnetic contact and the aluminum layer electrically joins the thick aluminum layers of the elements. The U beams that are compressed on top of the strips are, in addition, used to protect the joint, and also function as a supporting frame of the structure.
A magnetically shielded room can also be built in a more simple manner, that is, for example, from one aluminum plate or from two superimposed aluminum plates. As the material one can also use a mixture of iron and nickel (Fe—Ni) having a high permeability.
Two wall elements having a sandwich structure can be attached to one another by simply placing them on top of each other at one ends thereof over a short stretch and by pressing the joint tight e.g. with a bolt. The joint structure of this kind has the weakness that the joint is rough (having a “step” the size of the thickness of the wall) and susceptible to interference. The electrical conductivity between the aluminum parts of adjacent elements is not the best possible one due to the simple structure of the fastening.
One prior-art solution for improving the electrical conductivity of a junction between aluminum plates is to use separate strips. The strips form a contact between the aluminum plates so that the electrical conductivity in the junction is improved.
Publication U.S. Pat. No. 6,734,353 discloses a solution for compensating interferences in a magnetically shielded room. An interfering magnetic field is measured with coils placed in three different directions (directions of the X, y and z axis) that have each been wound about their own wall element. Based on the measurement results, a compensating magnetic field is generated in the measurement region.
The problem with the prior-art technique for all the solutions mentioned above is the weak contact of the aluminum plates. This results in a relatively strong coupling of the interference signals through the joints of the shielded room into the measurement room.